Some ideas:
- go to the presentations (if they have them) of 4th year students on their projects and see if anything they are doing interests you
- attent department seminars
- talk to your professors to see if they have any ideas for a 4th year project or if they remember any interesting projects that have been done over the past few years
- talk to others in your undergradute physics society and find out what ideas they have
- talk to graduate students in your department about the projects they are involved in
- start reading journal articles in fields that interest you
- read articles in publications like Science, Nature, and Physics Today